In the anisotropic high-rate plasma etching of silicon using a masking selectivity that is as great as possible, and profile control that is as good as possible, for example of the type in German patent reference no. 42 41 045, it is necessary to apply a stable passivation to the sidewalls of the structures to be etched, which may easily be removed again on the etching ground of the structures to be etched and while obtaining the masking selectivity, i.e. by an incidence of ions having a low energy input per unit of time. At the same time, it is necessary to make available a high density of etching species for the removal of the silicon on the etching ground.
Usually, fluorine radicals from fluorine supplying etching gases, such as SF6, NF3, ClF3, BrF3, etc, are used as etching species, which are broken up in a high-density plasma. As passivating species, there come into consideration, above all, teflon-forming monomers of passivating gases, such as C4F8, C3F6 or other fluorocarbons may have a low fluorine to carbon ratio, e.g. 2:1 or lower, which are also broken up in a high-density plasma.
The teflon-forming monomers build up a sidewall protective film, which prevents an etch attack on the sidewall, and leads to the desired anisotropy of the etching, while a directed ion incidence, above all on the etching ground, assures that the teflon-type protective films may be removed from there again, so that the etching ground essentially remains free from the protective film, whereas the fluorine radicals as etching species etch the exposed silicon surfaces on the etching ground.
If one makes the attempt to supply to a plasma fluorine-supplying etching gases and polymer-forming passivating gases at the same time, and to break these up in it, so as to generate simultaneously a high density of etching species, one observes a harmful mutual influence and undesired recombinations of the two species, i.e. instead of a buildup of polymer films on the sidewalls and efficient etching on the etching ground, the fluorine radicals and the polymerization-capable teflon former react to form saturated fluorine compounds which are largely inactive with respect to silicon.
In U.S. Pat. No. 5,498,312, in acceptance of this problem, it was suggested that one use an exceptionally high plasma density, in order to counter the undesired recombination reaction by using a correspondingly higher production rate of both species via a high density of both species which are incompatible per se. However, this attempt, in relationship to the achievable Si etching rate per kWatt of plasma efficiency, leads to relatively inefficient processes and is problematical with regard to profile control and the reactor contamination with polymers which appears in this context, which is caused, above all, by the required excess of passivating gas with respect to the etching gas.
An alternative attempt at a solution is referred to in U.S. Pat. No. 6,303,512, where etching gases and passivating gases are used which are more compatible with one another. Thus, in that document, SF6 or ClF3 is used as the fluorine-supplying etching gas, whereas the passivation is achieved by using oxygen and silicon tetrafluoride by depositing an SiO2-type of protective film onto the sidewalls of the structures to be etched.
Fluorine radicals and oxygen radicals or SiF4 do not react or recombine with one another, so that etching gases and passivating gases may be used without a problem as a stationary gas mixture. However, in this context it is disadvantageous that the sidewall passivating is effected by comparatively hard SiO2-type films, which require an increased energy input by a directed ion incidence, so that they can be broken down on the etching ground, which greatly reduces the masking selectivity. Consequently, the etching process according to U.S. Pat. No. 6,303,512 must be operated at maximum load, giving consideration to the masking selectivity, which increases the risk of undesired etching ground roughness and so-called “grass formation”. Passivating with the aid of SiO2 also has the disadvantage that the inhomogeneities of the energy input onto the etched substrate bring about far greater interference effects than in the case of teflon-type films.
German patent reference no. 42 41 045 solves the problem of the “unfriendly coexistence” of Si-etching fluorine radicals and monomers forming teflon-type films by separating their generation in time, or rather carrying it out in alternating fashion. In this manner, teflon-type films formed during so-called “passivating cycles” are removed again during subsequent “etching cycles” which are isotropic per se, and redeposited deeper into the generated trenches, so that a local protective effect is created by entrainment of the sidewall film into the depth of the trenches. In this context, the restricted plasma stability during the gas exchange is a problem, where impedance changes in the plasma may lead to a faulty adaptation of the coupled-in high frequency radiation or microwave radiation, which results in a reflected output right up to the interruption of the plasma discharge (“blinking”). In addition, using this method, there are individual cases where completely smooth sidewalls of the trenches produced are not achievable, which may be a disadvantage for optical applications using mirror surfaces.